AD8176 Datasheet, PDF (28/32 Pages) Analog Devices – 475 MHz, Triple 16 × 9 Video Crosspoint Switch

AD8176
( ) P_D,OUTPUT = V_POS − V_{OUTPUT, RMS} × I_{OUTPUT, RMS}
(5)
P_D,OUTPUT = (2.5 V− 1 V) × (1 V/100 Ω) = 15 mW
There are 27 output pairs, or 54 output currents.
nP_D,OUTPUT = 54 × 15 mW = 0.81 W
3. Subtract quiescent output stage current for number of
loads (54 in this example). The output stage is either
standing or driving a load, but the current only needs to be
counted once (valid for output voltages > 0.5 V).
( ) P_DQ,OUTPUT = V_POS − V_NEG × I_{OUTPUT,QUIESCENT}
(6)
P_DQ,OUTPUT = (2.5 V− (−2.5 V)) × 1.65 mA = 8.25 mW
There are 27 output pairs, or 54 output currents.
nP_D,OUTPUT = 54 × 8.25 mW = 0.45 W
4. Verify that the power dissipation does not exceed the
maximum allowed value.
P_{D,ON −CHIP} = P_D,QUIESCENT + nP_D,OUTPUT + nP_DQ,OUTPUT (7)
P_{D,ON −CHIP} = 3 W+ 0.81 W− 0.45 W = 3.36 W
From Figure 25 or Equation 1, this power dissipation is below
the maximum allowed dissipation for all ambient temperatures
up to and including 85°C.
In a general case, the power delivered by the digital supply and
dissipated into the digital output devices has to be taken into
account following a similar derivation. However, since the loads
driven by the H and V outputs is high and since the voltage at
these outputs typically sits close either rail, the correction to the
on-chip power estimate is small. Furthermore, the H and V
outputs are active only briefly during sync generation and
returned to digital ground thereafter.
Short-Circuit Output Conditions
Although there is short-circuit current protection on the
AD8176 outputs, the output current can reach values of 80 mA
into a grounded output. Any sustained operation with too many
shorted outputs can exceed the maximum die temperature
and can result in device failure (see the Absolute Maximum
Ratings).
Crosstalk
Many systems (such KVM switches) that handle numerous
analog signal channels have strict requirements for keeping the
various signals from influencing any of the other signals in the
system. Crosstalk is the term used to describe the coupling of
the signals of other nearby channels to a given channel.
When there are many signals in close proximity in a system, as
is undoubtedly the case in a system that uses the AD8176, the
crosstalk issues can be quite complex. A good understanding of
the nature of crosstalk and some definition of terms is required
Preliminary Technical Data
in order to specify a system that uses one or more crosspoint
devices.
Types of Crosstalk
Crosstalk can be propagated by means of any of three methods.
These fall into the categories of electric field, magnetic field,
and the sharing of common impedances. This section explains
these effects.
Every conductor can be both a radiator of electric fields and a
receiver of electric fields. The electric field crosstalk mechanism
occurs when the electric field created by the transmitter
propagates across a stray capacitance (for example, free space)
and couples with the receiver and induces a voltage. This
voltage is an unwanted crosstalk signal in any channel that
receives it.
Currents flowing in conductors create magnetic fields that
circulate around the currents. These magnetic fields then
generate voltages in any other conductors whose paths they
link. The undesired induced voltages in these other channels are
crosstalk signals. The channels that crosstalk can be said to have
a mutual inductance that couples signals from one channel to
another.
The power supplies, grounds, and other signal return paths of a
multichannel system are generally shared by the various
channels. When a current from one channel flows in one of
these paths, a voltage that is developed across the impedance
becomes an input crosstalk signal for other channels that share
the common impedance.
All these sources of crosstalk are vector quantities, so the
magnitudes cannot simply be added together to obtain the total
crosstalk. In fact, there are conditions where driving additional
circuits in parallel in a given configuration can actually reduce
the crosstalk. The fact that the AD8176 is a fully-differential
design means that many sources of crosstalk either destructively
cancel, or are common-mode to the signal and can be rejected
by a differential receiver.
Areas of Crosstalk
A practical AD8176 circuit must be mounted to an actual
circuit board in order to connect it to power supplies and
measurement equipment. Great care has been taken to create
an evaluation board (available upon request) that adds
minimum crosstalk to the intrinsic device. This, however, raises
the issue that a system’s crosstalk is a combination of the
intrinsic crosstalk of the devices in addition to the circuit board
to which they are mounted. It is important to try to separate
these two areas when attempting to minimize the effect of
crosstalk.
In addition, crosstalk can occur among the inputs to a
crosspoint and among the outputs. It can also occur from input
to output. Techniques will be discussed for diagnosing which
part of a system is contributing to crosstalk.
Rev. PrA | Page 28 of 32